Process for soldering using pre-fluxed solder powder

ABSTRACT

A solder powder is disclosed comprising minute particles of metallic alloy solder coated by a thin layer of rosin soldering flux such that the flux performs a triple function of: (a) insulating the individual alloy solder particles from electrical conductance; (b) adhesively holding individual particles of solder in place by virtue of the sticky characteristics of the preferred fluxes used to coat the solder powder; and (c) providing, when melted, an agent to clean substrates and workpieces to be joined by soldering.

United States Patent [191 Maierson et a1.

1541 I PROCESS FOR SOLDERING USING PRE-FLUXED SOLDER POWDER Inventors:Theodore Maierson, Dayton; Barry L. Roe, Jr., Centerville; James E.Williams, Kettering, all of Ohio The National Cash Register Company,Dayton, Ohio Filed: Dec. 1, 1971 Appl. No.: 203,923

Related U.S. Application Data Division of Ser. No. 35,414, May 7, 1970,Pat. No. 3,703,254.

Assignee:

References Cited UNlTED STATES PATENTS 11/1934 Parker ..228/56 1/1959Lawton .Q ..29/496X 1 I June 5, 1973 3,065,538 11/1962 Melchiors et a1..148/24 X 3,073,270 1/1963 Johnson et a1..." ..29/502 X 3,091,0295/1963 Davis et a1 ..29/502 X 3,172,385 3/1965 Varsone ..29/496 X3,418,179 12/1968 Raynes et a1. ..148/24 3,471,310 10/1969 Joseph etal.... ...117/100 M X 3,479,231 11/1969 Joseph 148/24 3,583,063 6/1971Growney 29/502 X 3,656,226 4/1972 Bume ..29/502 Primary ExaminerJ.Spencer Overholser Assistant ExaminerRona1d J. Shore A tt0rney E. FrankMcKinney and Robert J. Shafer [57] ABSTRACT A solder powder is disclosedcomprising minute particles of metallic alloy solder coated by a thinlayer of rosin soldering flux such that the flux performs a triplefunction of: (a) insulating the individual alloy solder particles fromelectrical conductance; (b) adhesively holding individual particles ofsolder in place by virtue of the sticky characteristics of the preferredfluxes used to coat the solder powder; and (0) providing, when melted,an agent to clean substrates and work pieces to be joined by soldering.

2 'Claims, 7 Drawing Figures PROCESS FOR SOLDERING USING PRE-FLUXEDSOLDER POWDER This is a division of application Ser. No. 35,414, filedMay 7, 1970, now U.S. Pat. No. 3,703,254.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention pertains to a prefluxed solder powder composition whereinfinely-divided particles of solder alloy are individually coated by athin coating of dielectric or electrically non-conducting fluxingmaterial. The present invention also relates to a process for effectingsoldered joints between metallic workpieces wherein free-flowing,flux-coated solder powder is positioned between workpieces to be joinedeither by being adhered to one of the workpieces or by being sprinkledinto a space associated with workpiece surfaces. The present inventionmore particularly pertains to a prefluxed solder powder or fluxingsolder alloy for use in fine soldering and soft metal brazing both inlarge-area, massive joints and for performing soldering and brazingoperations in hard-to-reach places and in repetitive, mass production,conditions. A preferred composition of the present invention comprisesparticles of solder of an alloy of lead and tin in low-meltingproportions and in size-from about five to about five hundred microns inaverage diameter.

2. Description of the Prior Art U.S. Pat. No. 3,172,385, issued Mar. 9,1965 on the application of Robert G. Varsane relates to flux-coatedpreforms of large solder masses useful in soldering metallic bodiestogether. The preforms of that patent are disclosed to have a length ofabout one-half inch and are said to comprise a sandwich type ofconstruction wherein a metallic workpiece to be soldered is coated bysuccessive, continuous, layers of solder and U.S. Pat. No. 3,389,116,issued June 18, 1968 on the application of Arthur W. Saha discloses thatminute metallic particles or pigments can be coated with dielectric orinsulating polymeric material in order to permit electrostatic sprayingof paints which include those metal particles or pigment as a portion ofthe coloring or tinting element therein. Such disclosure may beconsidered to be pertinent with reference to the present invention onlyin that an insulating layer is provided on metallic particles. There isnothing disclosed with regard to meltable particle coatings, nor is itdisclosed that the metals coated can be used as soldering or brazingalloys.

SUMMARY or THE INVENTION Heretofore, particularly to simplify solderingoperations wherein the metal pieces to be soldered together were smallor otherwise difficult to handle, bodies of a combination of. solder andflux have been used to increase efficiency of the operation. Such bodieshave been, characteristically, rather large compared to the area of thejoint to be soldered. Such large flux-metal pieces must bepre-manufactured and different pieces must be made for each differentjob in mass production or assembly-line fabrication. v

In some instances, it has been found to be required or desired toperform soldering operations in blind or hidden areas impossible toreach with conventional soldering equipment;-- such being a difficulttask prior to the present invention.

Also, in the manufacture of tiny electrical components, conductivemetallicpieces must be joined by means of tiny conductive leads whichmust be soldered cleanly with a minimum of solder overflow and with aminimum of residual flux and solder contamination at the joint. Acritical aspect of the above-mentioned difficulties resides in providingcorrect amounts of flux and solder, in the correct ratio, at the correctlocation, without the need to critical heating and cooling steps incompleting the soldering operation.

As is well understood in the artof soldering metallic bodies, varioussolder compounds may be used depend ing upon the nature of the metalsbeing soldered and depending upon the nature of the bond desired. Theword solder as used in the present disclosure, includes various types ofsolders comprising soft solders and low-temperature brazing materials.Preferred for use in the present invention are soft solders which arerelatively low-melting alloys of lead, tin, and copper and may includeantimony, solver, arsenic and bismuth, if desired or required to impartspecial properties. Special solders including other metals may be usedin practice of the present invention where electrical or other,

more specific, characteristics are required in the soldered bond.Previously and conventionally, soft solders were provided commerciallyin the form of wire or ingots and sometimes as powders. Wire solders aresometimes provided with a flux core.

In soldering operations, materials named flux at utilized as an aid tothe accomplishment of a successful metallic joint. Flux materialspromote the fusion of the solder and aid in cleaning the metal surfacesto be soldered. Fluxes also eliminate or prevent the formation ofcontaminating metallic oxides during the soldering operation. Fluxes arecommonly and preferredly melted at the intended joint just previous tomelting the solder so that the flux can perform its functionsimmediately prior to contact of the workpiece edges by melted solder incompletion of the joint. The use of flux during a soldering operationhas, inthe past, presented a considerable difficulty because the flux isgenerally commercially available in the form. of paste, jelly, liquid orsolid which is difficult to handle and to apply, particularly in correctamounts to inaccessible places with respect to delicate solderingoperations. Fluxes available for use in the present invention includethose materials which can be formed into a continuous, insulating,coating about individual, finely-divided, solder par ticles. Preferredfor use in the-present invention are acid-type organic polymeric fluxessuch as rosin and rosin-alcohol in its pure water-white form. Thesematerials provide excellent results, apparently because of theirproperties as a natural gum. The various types of preferred rosininclude gum rosin, wood rosin, and tall oil rosin, all resin acidschiefly of the abietic and primaric types. Being natural gums, theresins preferred in the present invention have adhesive propertieswhereby they can bond firmly to the solder particles and, in turn, formstrong adhesive bonds between the solder particles and any otherworkpiece substrate.

It should, however, be pointed out that fluxmaterials other than rosinmay be used, provided that such materials have the appropriate adhesiveor waxy" properties or provided that they can be combined with othermaterials such as rosin or other finely-divided tacky resinous bindermaterial'to obtain the adhesive properties, Thus, any flux is eligiblefor use, provided that it is a solid at room temperature, that it can bereduced to a finely-divided powder form, and that it can be coated ontothe individual solder particles as a continuous layer.

The individual, minute, particles of soldering material can be coated byany of several well known processes previously taught in the prior art.The individual particles can, for example, be coated by being suspendedin a solution of flux material dissolved in an evaporable organicsolvent. The suspension or dispersion of solder particles in that fluxsolution can then be cast in a drying pan or otherwise distributed, suchas by spray-drying, in order to evaporate the solvent and leave a thin,continuous, layer of flux material surrounding each individual solderparticle. The preferred method for coating the individual solderparticles is according to, en masse, encapsulating procedures whereinliquid-liquid phase separation is conducted in a capsule manufacturingvehicle utilizing a phase-separation-inducing material orphase-separation-inducing conditions and wherein the soldering flux ispresent as capsule wall material. Examples of the preferred method forcoating the individual solder particles will be set out below in orderto provide complete disclosure of a method for practicing the subjectinvention.

As mentioned above, the alloys most commonly and preferredly used inpractice of the present invention are so-called soft solder alloys andusually comprise tin and lead as major constituents. Also included ascomponent materials in some of the soft solders eligible for use inpractice of the present invention are rather minor amounts of metalssuch as antimony, silver and arsenic. The solders which can be used inthe present invention are differentiated, in the main, by difference inmelting point range and also by a difference in wetting capabilities aspertain to the various metallic workpieces to be joined. The mostpreferred solder alloy used in the present invention is a, so-called,eutectic solder which consists of about 38 percent, by weight, lead, 62percent, by weight, tin, and has a melting point of about 183centigrade. In practically all cases, no matter what solder alloy isused, the melting point for the solder will remain below a temperatureof about 425centigrade. An example of an eligible high temperaturesolder is an alloy containing about 97.5 percent, by weight, lead, 1.5percent, by weight, silver, and 1 percent, by weight, tin;--that solderalloy having a melting point of approximately 210 centigrade. An exampleof an eligible intermediate-temperature solder is an alloy includingabout 66 percent, by weight, lead, 32 percent, by weight, tin, 2percent, by weight antimony, and 0.1 percent, by weight, arsenis;suchalloy having a melting range of from about 186 to 241 centigrade.

A general process for practicing the present invention using flux-coatedsolder particles includes the following steps:

a. contacting flux-coated solder powder with a metallic substrate to besoldered wherein the metallic substrate has a surface temperature abovethe melting point of the flux and below the melting point of the soldermaterial;

b. cooling the metallic substrate to cause the fluxcoated solder powderto adhere to the substrate by fusion and resolidification of the fluxmaterial;

c. contacting the metallic substrate to be soldered with a metallicworkpiece to be joined thereto;

d. heating the contacting combination of metallic substrate and metallicworkpiece to a temperature at least above the melting point of thesolder alloy; and

e. cooling the contacting combination of metallic pieces to set thesolder and thereby create a soldered union of the pieces.

In the above-discussed process for practicing the present invention, thestep of contacting flux-coated solder particles with the substrate to besoldered includes any of several methods such as by immersing the warmedsubstrate to be soldered in a bed of flux-coated particles, eitherfluidized or not, to dissipate heat from the substrate into the bed ofparticles. In the process of heat dissipation, flux from the particleswill be melted and the particles will become adhered to the metallicsubstrate. Of course, the fluxing solder alloy powder can be sprinkledonto a cold substrate or workpiece which will eventually be solderedaccording to the process of the present invention and, in .that case,the solder powder will not adhere to the workpiece. In that case, caremust be exercised in maintaining an adequate amount of the flux-coatedsolder powder in the area of the intended solder joint. Anothervariation of the contacting steps in the process includes the instancewherein the solder powder might be sprayed onto a hot substrate andthere be instantaneously adhered by virtue of the flux being melted andstuck thereto. It is also possible that a substrate can be coated by anadhesive material which operates to adhere fluxcoated solder to thesubstrate. The above-mentioned order of contacting flux with thesubstrate and'then cooling the substrate can also be reversed whereinthe solder powder can be coated onto a substrate and then the substratecan be heated to melt the flux and cooled to solidify the flux andthereby adhere the solder particles.

Another process for utilizing the flux-coated solder of the presentinvention includes the following steps:

a. positioning flux-coated solder powder between substrates orworkpieces to be joined;

b. heating the substrates or workpieces to melt the flux;

c. continuing the heating to melt the solder and create a liquid solderbridge between the workpieces; and

d. cooling the assembly to complete the solder joint.

In the above-mentioned processes, the solder powder can be introducedbetween the substrates or workpieces to be joined by coating a metallicinterleaf on both sides with the coated solder powder and then insertingthe interleaf between metallic substrates to be joined and proceedingwith the steps of the process, as outlined above. The interleaf can beany metal which melts at a temperature higher than the solder and can becoated in the same manner as is described abovefor minute in size andshould range in size from about to about 500 microns in averagediameter. It has been found that solder particle sizes below about 10microns in average diameter do not permit application of the properamount of flux material to the individual particles.

It has also been found that fluxing solder alloy particles greater thanabout 500 microns in average diameter cannot properly adhere to thesurface of a substrate to be soldered.

Solder particles of small size eligible for use in the present inventioncannot be adequately covered by flux using the process of tumblingsolder particles and flux particles together in a dry pan. It isapparently true that the mass of the indivudal solder particles is notgreat enough to permit deformation, adherence and consequent coating bytiny particles of powdered flux material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically represents across-section of one, individual, flux-coated solder article of thepresent invention. Such article is usually referred to herein as aparticle. The flux-coated articles are used in a mass; but a figuresimply depicting a pile of the articles was deemed to have littleinstructive value and has been, therefore, omitted.

FIG. 2 is a schematic, corss-section, representation, in large-scale, ofa workpiece coated by solder powder composition of the presentinvention.

FIG. 3 represents a metallic part coated on two sides by the solderpowder of the present invention.

FIG. 4 represents a workpiece or substrate coated on one side by solderpowder of the present invention.

FIG. 4 is an exploded view, of two workpieces to be joined by aninterleaf coated by flux-coated solder powder.

FIG. 6 is similar to FIG. 5 with the exception that the flux-coatedsolder powder has been applied directly to one or both of the workpiecesand the interleaf has been omitted.

FIG. 7 is a schematic representation of two workpieces, for example,wire ends, which are to be joined according to the process of thepresent invention using the flux-coated solder powder.

In FIG. 1, the flux coating 10 is shown to becontinuous andsubstantially even over the entire surface of the solder particle 11 toyield the flux-coated solder particle 12 which, when used in a mass, iseffective according to the process of the present invention. FIG. 2schematically represents a-plurality of flux-coated solder particles 12covering one portion of a metallic workpiece 15 wherein the flux coating10 surrounding each solder particle 11, has been fused with eachadjacent coating 10 to yield a coherent whole. The flux-coated solderparticle 12 is shown in FIGS. 3 and 4 as a coating, en masse, onmetallic workpieces. In FIG. 3, the metallic part 13 is shown as being asmall metallic workpiece and, when coated, represents a powder coatedinterleaf l4 useful as before-described. In FIG. 4, the flux coatedsolder particles 12 are shown to be covering one portion of one surfaceof a metallic workpiece 15, FIGS. 5 and 6 demonstrate, in a schematicrepresentation, the manner in which elements from FIGS. 3 and 4 areutilized to practice the process of the invention. In FIG. 7, wire ends16 are depicted only as one of a multitude ofexamples which could beemployed to teach specific metallic workpieces eligible for use in thepresent invention. Of course in FIG. 7, the flux-coated solder powder 12can be applied over the entire surface of pieces to be joined or can beprovided on only a portion of the surfaces to be joined or can bepresent on the surface of only one of the two or-more pieces to bejoined. It should be noted that in all of the figures the workpieces,interleaves, and wire ends, are shown only as schematic representationsto more adequately and clearly explain the invention and set out theconcepts involved in practicing the invention and utilizing the coatedsolder powder. The drawings are not intended to limit the presentinvention and are to be viewed as disclosing the invention in concertwith the examples and the remainder of the specification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example I In the presentexample finely-divided particles of a eutectic solder comprising about 39 percent, by weight, lead and about 62 percent, by weight, tin werecoated as individual particles using an acid rosin designated by thetrademark K wood sold by Hercules, Inc., Wilmington, Delaware, U.S.A.

To prepare the coated solder powder composition,- 8 grams of the rosinwas dissolved in 392 grams of chloroform; such being selected to be anappropriate solvent for liquid-liquid phase separation of the rosin,which separation will be described! hereinbelow. To the solution ofrosin, was added 8 grams of the solder powder having an averageparticlesize range of about to about 250 microns, and the system was agitated tothe extent required to maintain complete dispersion of the solder powderin the solution. To this dispersion,

while maintaining agitation, was added, 4 milliliters ofl-hydroxyethyl-2-heptadecenyl glyoxalidine as surface active agent androsin plasticizer and the system was heated to about 60 centigrade.About 200 grams of silicone fluid (polydimethylsiloxane having amolecular weight indicated by a viscosity of 50 centistokes and sold,for example, by Dow Corning Corporation, Midland, Michigan, U.S.A. underthe trademark, DC- 200) were added to the system in a drop-by-dropmanner to accomplish phase separation of the rosin. Agitation of thesystem was continued and the temperature was permitted to decrease toabout 25-3() centigrade at which time about 2.5 milliliters oftetrabutyl titanate was added as chemical hardening agent for the rosincoating material. Agitation was continued for about three hours and thenabout 500 milliliters of cyclohexane was added as a liquid rinse to aidin separating coated solder particles from suspended solid residue.Agitation was stopped and the supernatant was decanted. The particleswere so-rinsed twice again using 500 milliliter portions of cyclohexaneand the particles were then dried to a free-flowing powder-like form.

It should be understood that the present invention is not considered toreside in the method for manufacturing coated solder particles but suchhas been disclosed to afford a more complete teaching.

In this example, the rosin-coated, free-flowing solder particles arecharged into an air-fluidized bed andthe metallic substrate is heated toabout 100 centigrade. The heated substrate is immersed into thefluidized bed, is cooled by operation of the bed and, in being cooledmelts the rosin on some solder particles which particles areconsequently adhered to the substrate. The now-solder-coated substrateis contacted with another metallic workpiece and the two pieces togetherare reheated;this time to a temperature above the melting point of thesolder. Those two pieces, including the melted solder between the piecesare then cooled to freeze or solidify the solder and complete thesoldered joint. This example can also be conducted wherein therosin-ocated solder particles are heated, in a fluid bed, to above therosin melting point and cool workpieces are contacted therewith.Agitation of the particles by being air-suspended, is necessary toprevent agglomeration.

Example 2 ln this example, the solder powder from Example 1 is coatedonto an interleaf by heating the interleaf and immersing it into a traycontaining the solder particles. That metallic interleaf is thenpositioned between two workpieces to be joined by soldering, theworkpieces are heated, cooled, and excess flux is cleaned from the jointusing an organic solvent such as acetone.

Example 3 -In this example, flux-coated solder particles are utilized inaffixing metal pegs to a metallic board having blind holes of a size toreceive the pegs. A portion of the solder particles are introduced intothe holes, the pegs are then inserted, and heat is applied to the extentrequired to melt, first the flux, and then the solder. The surfaces ofthe hole and the plug are first fluxed and then soldered.

Example 4 In this example flux-coated solder particles are applied to asubstrate to be soldered by utilizing a commercially available aerosolliquid flux preparation as a temporary adhesive material for joiningsolder particles to a substrate.

Although such use of liquid flux introduces a risk of excess fluxingmaterial, it avoids any inconveniences present in the fluid-bedapplication of Example 1, herein. A non-flux adhesive material can beutilized with only the adverse effect that the adhesive material mightcontaminate the soldered joint.

The liquid flux, as adhesive, can also be applied by methods other thanby spraying. As an example, the flux can be printed onto a metallicsubstrate in conformations or designs such that solder, when applied,will adhere in particular, desired areas.

Example 5 In this example, a layer of the flux-coated solder issandwiched between perforated metallic circuit boards which areselectively activated when caused to be electrically joined atparticularpoints. In the initial sandwich, the coated solder provides completeelectrical insulation between the circuit boards by virtue of thenon-conducting character of the flux material. Metallic pins or wireleads from another source are heated to a temperature above the meltingpoint of the solder and are inserted into appropriate performations inthe sandwhich. Heat from the pin or wire melts the flux, then melts thesolder, and provides electrical connection between both of the circuitboards and, in the case of a wire lead, the other source.

What is claimed is:

l. A process for manufacturing a soldered joint between metallicworkpieces wherein the soldering material consists essentially of amultitude of individual minute solder particles each covered by acontinuous coating of meltable flux material including the steps of:

a. contacting the meltable flux coated soldering material with thesurface of at least one of the metallic workpieces to be joined;

b. heating the workpiece surface to a temperature above that requiredfor melting the flux material but below that required for melting thesolder;

c. cooling the workpiece surface to solidify the flux and adhere thesoldering material, as a coated mass, to the workpiece surface;

d. contacting the workpieces with the solder-coating therebetween;

e. heating the workpieces to a temperature above the melting point ofthe solder; and

f. cooling the workpieces to solidify the solder and complete thesoldered joint.

2. A process for manufacturing a soldered joint between metallicworkpieces wherein the soldering material consists essentially of amultitude of individual minute solder particles each covered by acontinuous coating of meltable flux material including the steps of:

a. heating the soldering material, in an agitated, airsuspended state,to a temperature above that required for melting the flux material butbelow that required for melting the solder;

b. contacting the heated soldering material with a surface of at leastone of the metallic workpieces to be joined wherein the workpiecesurface temperature is below the flux material melting point and whereinthe flux material is thereby solidified and the soldering materialadhereed to the workpiece;

c. heating the workpiece surface to a temperature above the meltingpoint of the solder; and

d. contacting the workpieces to be joined with the soldering materialtherebetween to form a soldered joint.

2. A process for manufacturing a soldered joint between metallicworkpieces wherein the soldering material consists essentially of amultitude of individual minute solder particles each covered by acontinuous coating of meltable flux material including the steps of: a.heating the soldering material, in an agitated, air-suspended state, toa temperature above that required for melting the flux material butbelow that required for melting the solder; b. contacting the heatedsoldering material with a surface of at least one of the metallicworkpieces to be joined wherein the workpiece surface temperature isbelow the flux material melting point and wherein the flux material isthereby solidified and the soldering material adhereed to the workpiece;c. heating the workpiece surface to a temperature above the meltingpoint of the solder; and d. contacting the workpieces to be joined withthe soldering material therebetween to form a soldered joint.